Teach tos-storage-stm25p to derive NUM_SECTORS and SECTOR_SIZE.

The stm25p driver is suitable for different sizes of flash parts, such as the
commonly used 8Mb M25P80 and the less commonly used 1Mb M25P10A.  The driver
defines the flash configuration in Stm25p.h.  By having tos-storage-stm25p walk
the @includes for the platform, as found in the .platform file, each platform
can ensure that a Stm25p.h with correct content for its flash part is found
for use by the driver and this script.